1. Field of the Invention
This invention relates to a dual jacketed cable, and, more particularly, to a cable having a corrugated metallic layer overlying the core, an outer jacket of a material capable of withstanding elevated temperatures, and an inner jacket which is interposed between the metallic layer and the outer jacket and which is capable of withstanding effects of corrugation imprint.
2. Description of the Prior Art
In metropolitan areas it is not uncommon to run communications cable in underground ducts which are located adjacent to steam lines. Because the steam lines may have an adverse affect on the communications cable, it is incumbent upon cable manufacturers to provide a cable having a jacket which is capable of withstanding elevated temperatures.
In the past, polyethylene-jacketed, lead shielded cables were used in these environments. Not only was this arrangement very costly, but the outer layer of polyethylene, when exposed to high temperatures for a long period of time, tended to develop cracks. Cables having a polyethylene jacket extruded over a soldered seam steel shield have also been used. However, in cables of this latter construction, the soldered seam is not generally continuous. Since cables of this type are usually under a slight gas pressure, e.g., 10 p.s.i., the discontinuities in the sealed seam causes the gas pressure to be exerted on the polyethylene jacket which may cause degradation of the jacket.
One prior art design includes a cable core having a corrugated metallic layer for purposes of withstanding stresses due to the bending of the cable during installation, and a jacket comprised of a polybutylene material covering the metallic layer. While the polybutylene material was capable of withstanding those kinds of temperatures destined to be encountered in the underground duct system, it was also found that the polybutylene exhibits reduced stress-resisting capability because of the corrugation imprint from the metallic layer. As a result, it was not uncommon for the outer jacket to crack during the installation of the cable when it was bent in a curved configuration with rather sharp radius bends. The corrugation imprint caused the reduced thickness of the cable jacket to be rendered incapable of withstanding the stress encountered. This problem becomes more acute during cold weather installation since the mechanical properties of polybutylene begin to change below a temperature of approximately 40.degree. F.
Another one of the problems which was encountered in cable in which a single polybutylene jacket was applied directly over the metallic layer related to the curing time for the polybutylene which typically may be in the range of 14 days. Within 4 days, for example, the density changes from 0.88 grams/cc to 0.91 grams/cc. And since the polybutylene jacket engages is in engagement with the corrugations, it shrinks and becomes spaced from the corrugations, nonuniformly along the contour of the corrugations.
After the jacket is extruded over the corrugated metal layer and cooled, the cable is taken up on a reel which causes a tightening up of the cable. The curing of the polybutylene jacket after the cable is wound on the reel coupled with the pressure of the successive layers causes the corrugations of the metallic layer to penetrate further into the polybutylene if the polybutylene lies in direct engagement with the corrugated metallic layer. This exaggerated corrugation imprint results in localized thinness of the jacket adjacent the peaks of the corrugations with an accompanying tensioning of the polybutylene in the area of penetration.
Unfortunately, this further imprinting of the prior art single jacket, steam-resistant cables occured after the cable had been wound on the reel and hence subsequent to the conventional in-line jacket thickness testing. This lead to the anomalous situation where in line tests indicated an acceptable jacket thickness of a cable, but where at the point of the use the cable had unacceptable thin jacket.
Further, if, as is usually the case, the above-described cable is wound on a reel during the transition curing period of the polybutylene, the cable is said to develop a "reel set". The installation and attendant bending of a cable having "reel set" requires more strain with increased probability of jacket buckling. Moreover, the extrusion jacketing of a cable core establishes a weld line which tends to cause a longitudinal splitting of the jacket. The adverse effects caused by a polybutylene jacket contiguous to the corrugated metallic shield may aggravate this tendency to split longitudinally.
Because of the demand for this type cable in large metropolitan areas and because of the importance of maintaining the integrity of the cable during the installation and thereafter, efforts have been devoted toward overcoming the problems of exposure to elevated temperatures while maintaining the structural integrity of the cable during the bending and installation thereof.